1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system for internal combustion engines which are installed on automotive vehicles having automatic transmissions.
2. Prior Art
Conventionally, an automatic transmission equipped with a locking-up device is widely used for transmitting output torque from an engine to driving wheels of an automotive vehicle on which the engine is installed. The locking-up device operates to cause mechanical engagement between input and output members of a torque converter of the automatic transmission, when the automotive vehicle is in a predetermined travelling condition, to thereby reduce sliding loss of the torque converter.
An engine installed on a vehicle having such a automatic transmission equipped with a locking-up device is known, which employs means for setting the air-fuel ratio of a mixture supplied to the engine (hereinafter referred to as "the supply air-fuel ratio") to a predetermined value leaner than a stoichiometric air-fuel ratio when the locking-up device is in a directly coupled state (a state in which the input and output members of the torque converter are directly coupled with each other), to thereby reduce the fuel consumption, as disclosed e.g. in Japanese Patent Publication (Kokoku) No. 62-59218.
According to this manner of control of the air-fuel ratio, although the air-fuel ratio is controlled to the predetermined value leaner than the stoichiometric value, when the locking-up device is in the directly coupled state, it does not take into consideration a state in which the locking-up device effects loose engagement between the input and output members of the torque converter while allowing a slight degree of sliding therebetween. This results in an inconvenience of frequent changeover in the air-fuel ratio to be controlled, between the stoichiometric value and the predetermined leaner value, degrading the drivability of the vehicle.
Further, if the air-fuel ratio is always controlled to the predetermined value leaner than the stoichiometric air-fuel ratio, irrespective of the engine rotational speed or the travelling speed of the vehicle, the drivability can be degraded due to an insufficient output from the engine when the engine rotational speed is high, and surging (small vibrations of the engine caused by variation in output torque from the engine) can occur due to an unstable combustion of the mixture when the travelling speed of the vehicle is high.